1. Field of the Invention
The present invention relates to a noise suppression apparatus.
2. Description of the Related Art
It is known that an AM radio receiving device receiving an amplitude-modulated signal of AM radio broadcasting is affected by various kinds of noise such as sudden pulse noise and stationary noise (floor noise under weak electric fields, etc.), and accordingly, there are required various kinds of countermeasures against noise according to the nature of the noise.
There is proposed, as a noise countermeasure against pulse noise, for example, a mechanism of so-called noise canceller for detecting, the pulse noise to be superimposed on a received amplitude-modulated signal with a high-pass filter or the like and removing the detected noise by linear interpolation or temporary halt of an audio signal output (refer to Japanese Patent Application Laid-Open Publication No. 2007-28290, for example).
There is also proposed, as a noise countermeasure against stationary noise, for example, a mechanism for suppressing the stationary noise by detecting the stationary noise to be superimposed on an AM detected audio signal with a high-pass filter, etc. and attenuating the audio signal itself under muting processing (processing of temporarily stopping the audio signal output or making the audio signal at a certain low level) (refer to Japanese Unexamined Patent Application Laid-Open Publication No. 2004-56660).
However, for noise including noise corresponding to neither pulse noise nor stationary noise, such as noise having frequency intermediate between that of pulse noise and that of stationary noise (hereinafter, referred to as the target noise according to an embodiment of the present invention), there is no effective conventional noise countermeasure mechanism, where both noise reduction and sound quality improvement are compatible.
When using the above-mentioned noise countermeasure against pulse noise, for example, since the target noise of an embodiment according to the present invention is shorter in frequency and longer in noise width than the pulse noise, the target noise loses component of the original audio signal more than necessary by the linear interpolation or the like, thereby causing abrupt variation in audio signal, which may lead to sound degradation such as sound fade-out or intermittent sound.
For example, in the case of using the above-mentioned noise countermeasure against stationary noise, even though the target noise of an embodiment according to the present invention has a frequency higher than that of the stationary noise, since a high-pass filter allowing such a high frequency component to pass therethrough cannot be used for the AM detected audio signal in order to maintain sound quality. Therefore, the target noise cannot exactly be detected, so that effects of noise reduction may not be obtained.